The need for very wide signal bandwidth (˜10-100 MHz) radio frequency (RF) power amplifiers has significantly increased due to the emerging wireless communication technologies. A cost-effective solution is a single power amplifier, which can support existing communication technologies, such as Wideband Code Division Multiple Access (WCDMA), and Global System for Mobile Communications (GSM), and Enhanced Data rates for GSM Evolution (EDGE) in one frequency band as well as new Long-Term Evolution (LTE) technologies in other frequency bands. However, there are many design challenges for a low-cost implementation of a high-performance very wideband RF power amplifier.
Average power efficiency is of particular importance in wireless communication technologies that transmit a very high peak-to-average power ratio (PAPR) signal. In conventional linear PAs, the average power efficiency for a high PAPR signal is significantly lower than peak power efficiency because the power efficiency is basically proportional to the output power. For example, if the peak power efficiency is 60% at the peak output power, the power efficiency at 6-dB output power back-off is only 30% with an ideal Class-A PA, exhibiting more than 50% loss. The active element remains conducting all of the time. For concurrent dual-band LTE signals with 40-MHz aggregated bandwidth, which have higher than 10-dB PAPR, the average efficiency of an ideal Class-A PA is only approximately 6%.
FIG. 1 shows a prior art envelope tracking PA, which significantly improves the average power efficiency for high PAPR signals, see e.g., U.S. Pat. No. 8,737,940, U.S. Pat. No. 8,626,091, U.S. Pat. No. 8,600,321, and JP 2011109233. The envelope tracking power amplifier systems typically include a direct current (DC) bias network composed of 101 (DC block capacitor) and a RF choke 102, an RF choke inductor 103, a main RF PA 120, and envelope amplifier 110.
An envelope signal amplitude 106 (0-2VPA) is extracted from a digital baseband of the RF signal 104 or directly from an analog RF signal, while an input signal 105 is fed to the envelope amplifier 110 to modulate the PA supply voltage 107. Because the envelope amplifier 110 dynamically modulates the PA supply voltage, the PA 120 always provides maximum output power at a given supply voltage. Therefore, the average power efficiency of an ideal envelope tracking power amplifier is theoretically the same as the peak efficiency of the main power amplifier, which is the key advantage.
However, envelope tracking power amplifier systems have extremely challenging design requirements for the envelope amplifiers that must operate with high power efficiency and wide bandwidth. Compared to conventional linear PAs, the disadvantage of the envelope tracking PAs is the limited bandwidth and efficiency that arises from the envelope amplifiers. Although there are efforts to improve the design trade-off of the envelope amplifiers between output power and bandwidth, e.g., see U.S. 20130217345, U.S. 20130200865, and U.S. Pat. No. 6,043,707, it is very challenging to transmit RF signals with a channel bandwidth higher than 20 MHz with the envelope tracking PAs. Another disadvantage of the envelope tracking PAs is the increased implementation cost and form factor due to the complexity of the envelope amplifier 110.
FIG. 2 shows a prior art self-envelope tracking PA, which removes the envelope amplifier by using an input network 200 and self-envelope load network 210. Therefore, the self-envelope tracking PA shown in FIG. 2 can achieve a significant reduction in both cost and size compared to the envelope tracking PA shown in FIG. 1.
The operation of the self-envelope tracking PA is as follows. An RF input signal 205 is applied through a DC blocker 201. The envelope amplitude of the RF input signal is inverted 206, and then applied through another DC blocker 203. The RF input signal 205 is prevented from passing towards the DC supply by using an inductor 202, which provides high impedance at the RF frequency. The gate bias of the main PA 220 is established by a resistor 204, which isolates the DC gate bias from both the RF input signal 205 and the envelope signal 206. The envelope signal 206 modulates the PA supply voltage 213 at the node between the RF choke inductor 221 and the inductor 212 of the self-envelope load network 210. The resonant frequency of the LC tank, which is formed by the capacitor 211 and the inductor 212, is tuned to the frequency where the CDF (cumulative distribution function) of the signal is approximately 50%. Another advantage of the self-envelope tracking PA is that a low supply voltage can be used; the PA output signal 222 can be obtained by using a supply voltage Vdd, which is lower than the supply voltage Vpa of the conventional envelope tracking PAs, further improve the system power efficiency.
FIG. 3 shows one disadvantages of the prior art self-envelope tracking PAs. Because the self-envelope load network 210 has a resonant frequency f0, the power efficiency of the self-envelope tracking PAs 300 is highest at the resonant frequency f0. Therefore, the bandwidth of the self-envelope tracking PAs is limited. Beyond the cross-over frequency fc, the power efficiency of the self-envelope tracking PAs 300 becomes even lower than the power efficiency of the fixed voltage supplied PA. With the conventional self-envelope load network 210, the cross-over frequency fc is typically below 10 MHz.